So my question is do I actually need that specific input? What does it do? I also plan on connecting two turntables and realize that I will need a mixer to do that. Will that change things?
The output from a phono pickup cartidge is about 4-5 mV. The output from most any other line-level source is much higher, about 100-200 mV. So you need a phono preamp to boost the signal and to equalize it properly so the sound is balanced and not tinny sounding. Most older receivers, like yours, had this built in and were fed signal by the phono input jack.
If you have moving magnet (not moving coil) pick ups on your turntables, get an inexpensive phono preamp like the
Audio Technica PEQ3. You can plug it into any RCA jacks on a receiver that accept standard line-level input signals, such as a CD or DVD player.